Fabrication of integrated circuits includes the formation of patterned layers on a semiconductor wafer that form electrically active regions in and on the wafer surface. As part of the manufacturing process, a masking process referred to as photolithography or photomasking is used to transfer a pattern onto the wafer. Masking involves applying a photoreactive polymer or photoresist onto the wafer by any suitable means such as by spinning of the wafer to distribute liquid photoresist uniformly on its surface. In a typical semiconductor manufacturing process, several iterations of the masking process are employed. Layers of either positive or negative photoresist can be used in various combinations on the same wafer.
Typically the wafer is heated or “soft baked” on a hot plate to improve adhesion of the resist to the substrate surface. A photo aligner aligns the wafer to the photomask and then portions of the resist coated wafer are exposed to high-energy light so that a pattern is formed as a latent image in the resist layer. A developing agent is then applied to develop the portions of the resist which were exposed. When positive resist is used, the developed portions of the resist are solubilized by the exposure to high-energy light. Conversely, when negative resist is used, the undeveloped portions of the resist are solubilized. Washing and rinsing steps are carried out that selectively remove the solubilized photoresist. A drying step is carried out.
Pattern formation using the traditional lithographic techniques has been carried out using an aqueous solution of tetramethyl ammonium hydroxide (TMAH) as the developer. For example, Takizawa, in U.S. Pat. No. 6,472,127, issued Oct. 29, 2002, discloses a method of forming a resist pattern, including a develop step performed by applying an aqueous solution containing 2.38% TMAH as an alkali developer to the photoresist film. Nishi, et al., in U.S. Pat. No. 6,492,090, issued Dec. 10, 2002, discloses a polymer, a resist composition, and a process for forming a resist pattern. In the method according to Nishi, et al., development is carried out using as the developer an 0.1 to 5% (preferably 2 to 3%) aqueous solution of TMAH, this being done by a conventional method such as dipping, puddling, or spraying.
While mainly used as a developing agent, TMAH has been used as an etchant in semiconductor manufacturing processes. For example, Wu, et al., in U.S. Pat. No. 6,461,967, issued Oct. 8, 2002, discloses a method of removing material from a semiconductor substrate, which includes an etchant containing TMAH to etch silicon-containing material. Makiyama, et al., in U.S. Pat. No. 6,509,252, issued Jan. 21, 2003, discloses a method of manufacturing a semiconductor device, which includes an etching process wherein the etchant is an aqueous solution of TMAH.
It is known that TMAH can be used to assist in the cleaning step of semiconductor manufacturing processes. For example, Nguyen et al., in U.S. Pat. No. 5,597,983, issued Jan. 28, 1997, discloses a method for removing a via sidewall polymer residue formed incident to an etch, wherein a resist developer containing TMAH is used to remove the polymer residue from the via. Chang, et al., in U.S. Pat. No. 6,232,238, issued May 15, 2001, discloses a method for preventing corrosion of a bonding pad resulting from residual polymers on a surface of a semiconductor wafer. In the method according to Chang, et al., an alkaline developer comprising 2.38% TMAH is used for removing the residual polymers.
While TMAH can be used in various steps of semiconductor manufacturing processes, generally it is contained in an aqueous solution. For example, the traditional develop methods utilize a premixed solution of TMAH dissolved in water and dispensed onto the wafer through differently designed nozzles. This leads to distribution, timing issues, and other problems across the wafer that make it difficult to control critical dimensions and other develop defects. When water is used for rinsing, e.g., for resists developed in an aqueous solution of TMAH, the presence of moisture in the atmosphere can not be avoided. This presents a problem because moisture in the atmosphere can cause acrylate-type resist to swell and pattern deformation can occur. The use of water as the developer solvent and rinse agent causes problems because of its high surface tension, ˜72 dyn/cm, when compared, e.g., to the surface tension of methanol, ˜23 dyn/cm.
It is known that capillary forces scale with the surface tension of the rinse solution. A serious problem emerges when the mechanical stability of the resist lines is too weak to compensate for capillary forces exerted by the rinse liquid during the drying step. During drying, unbalanced capillary forces exert a net force on the pattern that tends to deform the resist lines. When the capillary forces exceed the elastic restoring force of the polymer, collapse of the photoresist structure occurs. As noted in the literature, collapse of photoresist structures is a generic term that refers to the deformation (bending), fracture, and/or peeling of resist from the substrate, in response to capillary forces present during the drying stage of a lithographic process. D. Goldfarb et. al, Aqueous-Based Photoresist Drying Using Supercritical Carbon Dioxide to Prevent Pattern Collapse, J. Vacuum Sci. Tech. B 18(6), 3313 (2000).
This problem of collapse of photoresist structures is exacerbated by high-aspect-ratio resist patterns that are desired as the minimum device feature size decreases. As device feature sizes shrink, the thickness of the resist is constant while the width of the pattern decreases. This results in a higher aspect ratio of height to width of resist lines. In general, as the aspect ratio increases, the mechanical stability of the resist lines decreases. The collapse of high-aspect-ratio photoresist structures is related to the surface tension of the rinse solution (capillary forces scale with the surface tension of the rinse solution) and is a function of both the density (spacing) and aspect ratio of resist lines. The collapse of high-aspect-ratio photoresist structures becomes an increasingly serious problem as semiconductor device feature sizes continue to shrink while relative vertical height increases to accommodate more complex interconnect structures.
There is a need for effective methods for developing the resist layer with no pattern collapse or deformation of the photoresist.